Immunonologic studies on hepatitis A virus have concentrated on T-cell immunityand experimental vaccinology using the chimpanzee model. Chimpanzees were immunized with the expression product of a recombinant vaccinia virus containing the entire open reading frame of the HAV genome. In appropriate cell culture antigenic particles have been produced. These particles were purified and adsorbed to alum as an experimental vaccine. Two chimpanzees were immunized and one was held as an uninoculated control. Both animals that received the vaccine developed antibody that was apparent even after the first dose. Upon challenge, the uninoculated animal seroconverted but unfortunately did not develope overt hepatitis. The vaccinated animals also appeared to be completely protected but as the control animal was infected without disease it is not possible to prove that they were protected. Chimpanzee T-cell lines specific for hepatitis A have been established and partially characterized. A cytotoxic T-cell clone has been established and several T-cell lines specific for hepatitis A have been selected. Both cytotoxic and helper activity are being studied and the epitopes to which these cells react are being evaluated.